In transmission systems using wavelength multiplexing, a plurality of channels can be conveyed simultaneously along a common fiber, each of the channels being carried by an associated wave having a determined center wavelength and a determined bandwidth. For performing routing or directing functions, it is often necessary to drop certain channels and to modify the multiplex, e.g. so as to modify the information in one or more channels or to replace the information-carrying wavelength with another wavelength.
To solve this problem, a conventional solution consists in using a demultiplexer whose outlets are connected to a multiplexer via switching means for selectively dropping and adding one or more wavelengths of the received multiplex. Unfortunately, that solution suffers from the drawback of only being capable of operating with a multiplex formed of precisely determined wavelengths. Another drawback is that an active element, namely the switch, is inserted in the main path between the inlet and the multiplexed outlet of the apparatus. All of the signals are thus interrupted while the switch is changing state, or when the switch fails.
Other solutions exist using, for example, tunable filters or fixed filters associated with switches. All of those solutions also suffer from the drawback of inserting an active element in the main path.